I'd say that the very small number of carbines out there that get hard use (Mil, LE, hard use in training courses, etc) need a BUIS, just in case.

Otherwise, the vast majority of carbines that are tricked out just look cool in for photo shoots and buddy get togethers, 100 rounds per year from the bench, and regular rubdowns with a cloth diaper... Well those don't need BUIS per se....

And by the way, the Magpul BUIS is cheap, light, and reliable. I wouldn't personally drop big bucks on a BUIS when the Magpuls do the same job cheaper and lighter and just as well.